1. Part III: Adjusting Flow-cycle Criteria in PSV When Using a Critical Care Ventilator for NPPV
By: Susan P. Pilbeam, MS, RRT, FAARC
John D. Hiser, MEd, RRT, FAARC
Ray Ritz, BS, RRT, FAARC
American Association for Respiratory Care
December, 2006

2. Section Objectives
After reviewing this section, the participant will be able to:
Describe how the ventilator ends a breath in pressure support ventilation
Explain how the flow curve during a pressure support breath varies depending on the lung pathology
Recommend to the flow-cycle value in patients with COPD who are actively exhaling
Suggest a new flow-cycle value if a large leak is present during PSV

3. Problems with PSV During NPPV
Using critical care ventilators for NPPV can be problematic
Large leaks may prevent cycling in some ventilators
Some will auto cycle the breath, some will not
Setting high flow-cycle criteria can reduce volume delivery
This section will look at how to set flow cycle percentage during pressure support with NPPV

4. Characteristics of a Pressure Support Breath
A pressure support breath is patient triggered. (a patient’s inspiratory effort begins inspiratory gas flow)
It is pressure limited. The pressure level goes to the pressure value set by the operator during inspiration.
And, it is flow-cycled. The inspiratory flow ends when the ventilator detects inspiratory flow has dropped to a specific flow value.

5. Characteristics of a Pressure Support Breath
In this pressure-time curve, arrow “A” marks the patient’s inspiratory effort
Arrow “B” marks the set pressure
Notice there is no arrow “C” to indicate the flow terminating criteria
We need to examine a flow-time curve to see how flow-cycling works
Pressure A

6. Flow-cycling and PSV
100 75 50 25
The flow time curve to the right illustrates flow cycling
Peak inspiratory flow is 100 L/min
The ventilator is set to flow-cycle the pressure support breath at 25%
Flow in L/min
Notice that flow rises rapidly at the beginning of inspiration. The peak flow reaches 100 L/min. This value is used only because it is easy to do some calculations for examples that we will be giving. The inspiratory flow from the ventilator ends the pressure support breath when the ventilator detects that the flow has dropped to 25% of the peak flow.
Time

7. Purpose of Flow-Cycling
The purpose of flow-cycle is to end inspiration when the patient is about to stop inhaling
We know this because the flow drops off toward the end of inspiration
The flow at which a ventilator cycles into exhalation can be a fixed value or it may be a value selected by the operator.

8. Examples of Fixed Flow-Cycle
For example, on some ventilators the flow cycle is set at a constant value of 25% of peak inspiratory flow
On at least one other ventilator the flow cycle is set at 5 L/min…not a percentage, but a specific flow value

9. Flow-cycling and PSV
In newer ventilators this parameter is an adjustable control
The flow-cycling variable is adjustable anywhere from 1% to 80% of the measured peak inspiratory flow
The exact range depends on the ventilator

10. Names for Flow-Cycle Variable
The flow-cycling variable is given different names depending on the ventilator in use
Some example names are –
Inspiratory cycle-off
Inspiratory flow termination,
Expiratory flow sensitivity,
Inspiratory flow cycle %,
E-cycle etc…
The name varies with each ventilator

11. Different Settings for Flow-Cycle
100 75 50 25
Using the flow-cycle control allows the RT to vary the setting depending on the leak or on the patient’s desired inspiratory time.
We will look at a few examples of how changing the flow-cycle variable affects delivery of a pressure support breath.

12. Flow Cycle Percentage and Inspiratory Time in PSV
100 l/min
40 l/min
25 l/min
This graph provides two examples. Both of these patients have the same peak inspiratory flow rate of 100 L/min. The ventilator was set to flow-cycle at 40% in patient 1 and at 25% in patient 2. The ventilator measures the gas flow during inspiration and identifies the peak inspiratory flow. In this case the peak flow for both patients is is 100 L/min. The ventilator then multiplies the set flow-cycle % times the peak inspiratory flow. For example, with patient 1, 40% of 100 L/min is 40 L/min. The ventilator ends the pressure support breath when the flow drops to 40 L/min. In patient 2, 25% of 100 L/min is 25 L/min. The ventilator ends the pressure support breath when inspiratory flow drops to 25 L/min. Notice that Patient 1’s PS breath is SHORTER than patient 2’s.
The higher the flow-cycle percent that is set, the shorter the PS breath. If it is set very high (70%), it might compromise volume delivery for that pressure support breath.
Patient 1
Patient 2

13. Effects of Flow-Cycle %
A lower percentage gives a longer breath
A higher percentage gives a shorter breath
Using a higher percentage can reduce the tidal volume delivery
How can the RT correct the volume delivery when the flow-cycle is set high?
The answer is to increase the pressure support level.

14. Where Do We Set Flow-Cycle?
First, flow-cycle is set based on patient pathology
In patients who have increased airway resistance, we might want to use a higher flow cycle, such as 40%
In patients who have decreased compliance, we might want to use a lower flow cycle
The next few slides will examine why we do this

15. The Flow Curve
Inspiratory flow determined by set pressure and patient effort
100 L/min
25 cm H2O
The next few slides demonstrate how the flow curve is “created” during a pressure support breath. At the beginning of inspiration, the pressure in the ventilator is at the value that is set. In this example it is 25 cm H2O. Pressure in the patient’s alveoli is zero. The pressure gradient is 25 minus 0 or 25 cm H2O. The gradient is always the highest at the beginning of the breath. Remember that the higher the pressure gradient and the shorter the Rise Time the faster the gas flow. Flow increases very rapidly at the very beginning of the breath.

16. The Flow Curve
Inspiratory flow determined by set pressure, rise time and patient effort
100 L/min
25 cm H2O 18
As the lung fills, the pressure gradient decreases. In this example, the set pressure is 25 cm H2O and the alveoli are now at 18 cm H2O. The pressure gradient is only 7 cm H2O (25 – 18 = 7). With the lower pressure gradient, the flow slows down.

17. The Flow Curve
Inspiratory flow determined by set pressure, rise time and patient effort
100 L/min
25 cm H2O 19
% flow for
cycling
Finally the flow drops to the value that is identified by the ventilator as the flow-cycle point and the breath ends. In normal lungs, the flow curve drops fairly rapidly after it has reached its peak.

18. Patients With Increase Airway Resistance and Reduced Compliance
With high airway resistance and low compliance – a long time constant
Longer, slower flow curve
25 cm H2O
Flow
Patients with increased airways resistance and reduced compliance have long time constants. Their lungs fill more slowly. At the beginning of the breath in this example, the ventilator is giving 25 cm H2O of pressure and alveolar pressure is zero. The flow is at its highest value (highest pressure gradient) at the beginning of inspiration just as with normal lungs.

19. Where Do We Set Flow Termination?
Longer, slower flow curve
Use a higher flow termination to allow for adequate exhalation time
25 cm H2O 15
40% of
Peak flow
However, with increase airway resistance, it takes longer for the alveolus to receive the gas, so the pressure gradient between the ventilator and the alveoli changes more slowly than with normal individuals. The descending flow curve is more flattened than it would be in normal airways.

20. PSV and COPD
Patients with COPD commonly have a longer expiratory time due to their disease process
They also use their accessory muscles more than normal individuals
They tend to be active breathers
This creates another problem with PSV

21. Pressure-Time Curves in a Patient With COPD on PSV
The pressure spike at the end of inspiration is caused by the patient trying to actively exhale
Pressure
cm H2O
Pressure Spike
Normal Breath
In a patient with COPD who is receiving pressure support ventilation, it is not uncommon to see a peak in the pressure-time curve at the end of exhalation. Normally the pressure curve is flat during inspiration. The end-inspiratory peak that occurs is a result of the patient actively beginning to exhale into the ventilator circuit, thus increasing pressure in the circuit. The patient wants to breath out (exhale) sooner that the ventilator is allowing him/her to do so. To correct this situation the RT can shorten the PS breath by increasing the flow-cycle percentage. Be CAUTIOUS in doing this because it might reduce the delivered tidal volume. The RT may also need to increase the pressure to compensate.
Time

22. One More Potential Problem
One more difficulty may occur when using NPPV with a critical care ventilator set for pressure support breaths--
When there is a large leak in the system, inspiratory time may be prolonged
If the leak is large, the flow might never decrease to the flow-cycle level that is set because gas keeps escaping and the ventilator continues the flow

23. For Example
If peak inspiratory flow is 100 L/min and the flow-cycle is set at 25%, PS inspiration will normally end when the inspiratory flow drops to 25 L/min
However, if there is a leak in the circuit large enough to allow 35 L/min to flow out of the circuit, then the ventilator will never see the flow drop to 25 L/min and inspiration is prolonged
All current ICU ventilators have a maximum time that is allowed for inspiration in this situation. Usually this is between 2 and 5 seconds. In other words, if the ventilator never sees the flow drop, it will still end inspiration after a certain number of seconds has elapsed.

24. Ventilator Response to the Large Leak Problem in PSV
In this instance, time will end the breath
Most ventilators have an ‘I-time too long’ alarm to alert the clinician of this situation
The breath ends when the maximum time criteria is detected by the ventilator

25. To Solve the Problem When a Leak is Present
The RT can eliminate or minimize the leak or set a higher cycling percentage
Using the previous example…
If the RT sets the flow-cycle at 45% or 50%, then the ventilator will be able to detect the drop in flow and end the breath
Remember from the example the leak was 35 L/min (peak inspiratory flow 100 L/min)

26. Section Summary Let’s review what we learned in this section
How the ventilator ends a breath in pressure support ventilation?
How the flow curve during a pressure support breath varies depending on the lung pathology?
When to adjust the flow-cycle value in patients with COPD who are actively exhaling?
How to readjust the flow-cycle criteria if a large leak is present?